1. Field of the Invention
The present invention is for a method of preparing a surface of niobium and more particularly a method of preparation for high quantum efficiency niobium surface.
Radio frequency, photocathode, electron guns are the source of choice for most high-performance accelerator systems. The main reason for this popularity is their ability to produce very bright beams of electrons. However, due to inherent limitations, photocathode radio frequency electron guns have not successfully penetrated certain key applications. One of these limitations is their inability to economically produce the high average current, high brightness electron beams necessary for certain applications. Another drawback is that one must choose between high quantum efficiency and durability. Durable cathodes tend to have relatively low quantum-efficiency, while high quantum efficiency cathode materials are very sensitive to vacuum conditions.
Superconducting Radio Frequency injectors are highly sought after for high brightness, high duty factor electron sources to be used in Free Election Lasers and for election cooling of ions. The major hurdle in its development is the lack of a suitable photocathode that has high quantum efficiency, long life time and compatible with the superconductivity of the injector. This surface preparation method for niobium increases the quantum efficiency from 10xe2x88x926 to 10xe2x88x923. This makes the construction of superconducting radio frequency injectors with niobium as the photocathode feasible.
2. Description of the Prior Art
U.S. Pat No. 5,923,045 to Nihashi, discloses a semiconductor photocathode. Nihashi teaches use of a semiconductor p-type materials in layers to comprise a photocathode which, with an externally applied voltage, accelerates and emits an electron generated in response to light incident. Examples of material for the third semiconductor layer include combinations of Csxe2x80x94O, Csxe2x80x94I, Csxe2x80x94Te, Sbxe2x80x94Cs, Sbxe2x80x94Rbxe2x80x94Cs, Sbxe2x80x94Kxe2x80x94Cs, Sbxe2x80x94Naxe2x80x94K, Sbxe2x80x94Naxe2x80x94Kxe2x80x94Cs, and Agxe2x80x94Oxe2x80x94Cs. Nihashi does not suggest, disclose or teach the use of Nb as the semiconductor. Further, Nihashi only says the semiconductor xe2x80x9cis preparedxe2x80x9d, or xe2x80x9cetchedxe2x80x9d. There is no disclosure of a method to prepare the layers.
U.S. Pat No. 3,939,053 to Diepers, discloses an apparatus for polishing of niobium structures. This apparatus is arranged for partial immersion in an electrolyte bath of H.sub.2 SO.sub.4, HF and H.sub.2O. Diepers does not suggest or teach a mechanical way to polish a sample. U.S. Pat No. 4,014,765 to Roth, discloses a method for electrolytic polishing of the inside surface of hollow niobium bodies.
U.S. Pat No. 4,266,008 to Kampwirth, discloses a method for etching thin films of niobium and niobium-containing compounds for preparing super conductive circuits. Kampwirth teaches contacting the film with an aqueous etchant of 8 to 10 w/o HNO.sub.3, 11 to 13 w/o H.sub.2 SO.sub.4 and 12 to 13 w/o HF for a period of time to remove the unmasked super conductive film from the substrate; and removing the photoresistive material from the surface of the super conductive film, thereby forming a thick film superconducting circuit on a non-conductive substrate. Kampwirth does not suggest or teach a mechanical way to polish niobium.
An object of the present invention is to provide a method to prepare niobium such that it is a high quantum efficiency material in order to be used as an efficient superconducting electron source.
A method of surface preparation of a chemically etched and cleaned niobium surface to produce a high quantum efficiency material by polishing includes removing coarse scratches and fine scratches. Then rinsing the niobium surface and cleaning the niobium surface to remove polishing material. The niobium surface may be blown with high purity nitrogen prior to cleanly transferring the niobium surface into a vacuum chamber. The chamber is pumped until background pressure is low and the niobium surface is baked. Lastly, the niobium surface irradiated with a laser.
A preferred form of the method, as well as other embodiments, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments thereof.